is a collection of community-made levels included on the "Master Levels for Doom II" CD released by Id Software in 1995.
The collection was Id Software's answer to the various Doom level collection CDs that had begun to circulate around the time. The CD contained 21 levels selected and exclusively licensed by Id (the Master Levels) and a larger collection of 1830 other levels of varying quality (Maximum Doom).
The complete contents of the CD are not included here: the Master Levels are copyrighted to Id. This is just the levels from the "Maximum Doom" collection, which should all be freely redistributable. Many of the WADs are missing their accompanying text files. I'm just including what's on the CD.
To browse the full collection of WADs, see here